Fluid systems frequently require the supplementation of an additive to a pressurized carrier liquid, with the additive/carrier ratio being constant, predetermined and independent of the instantaneous parameters of pressure and flow in the system.
In systems with pipelines up to a diameter of 1″, this is achieved by means of proportional injection pumps of the hydraulic in-line type. In larger systems, use is made of electro-mechanical systems in which the injection rate of on-line hydraulic pumps is computer-controlled.